Fixing attributes to compile with GCC 4.8

[Gregwar]

The correct toolchain is now available in debian sid/testing:
https://packages.debian.org/jessie/gcc-arm-none-eabi
https://packages.debian.org/jessie/libstdc++-arm-none-eabi-newlib

I've successfully compiled LibMaple with that. However, there is two errors here:
https://github.com/leaflabs/libmaple/blob/master/libmaple/include/libmaple/libmaple_types.h#L58

First, __unused is already defined, a testing it with ifndef is enough to fix that

The __always_inline is also defined, but it doesn't automatically adds the inline keyword. So, I propose changing all the occurences of __always_inline in the lib maple to inline __always_inline and also adding an ifndef

I can do a PR for that

[Gregwar]

See:
RobotsWar/RobotsWar@af4d4ba

[mbolivar]

Fixed, thanks; please reopen if you run into trouble.

[mbolivar]

(I think the extra "inline" with __always_inline is redundant: https://gcc.gnu.org/ml/gcc-help/2007-01/msg00051.html)